1. Field of the Invention
The present invention relates to waveguide circulators, and more particularly to improved power handling capabilities for ferrite waveguide circulators through the use of thermally-conductive dielectric attachments.
2. Description of the Related Art
Ferrite circulators have a wide variety of uses in commercial and military, space and terrestrial, and low and high power applications. A waveguide circulator may be implemented in a variety of applications, including but not limited to transmit/receive (T/R) modules, isolators for high power sources, and switch matrices. One important application for such waveguide circulators is in space, especially in satellites where extreme reliability is essential and where size and weight considerations are very important. Ferrite circulators are desirable for these applications due to their high reliability, as there are no moving parts required. This is a significant advantage over mechanical switching devices.
A commonly used type of waveguide circulator has three waveguide arms arranged at 120° and meeting in a common junction. This common junction is loaded with a non-reciprocal material such as ferrite. When a magnetizing field is created in this ferrite element, a gyromagnetic effect is created that can be used for circulating the microwave signal from one waveguide arm to another. By reversing the direction of the magnetizing field, the direction of circulation between the waveguide arms is reversed. Thus, a switching circulator is functionally equivalent to a fixed-bias circulator but has a selectable direction of circulation. Radio frequency (RF) energy can be routed with low insertion loss from one waveguide arm to either of the two output arms. If one of the waveguide arms is terminated in a matched load, then the circulator acts as an isolator, with high loss in one direction of propagation and low loss in the other direction.
Generally, these three-port waveguide switching circulators are impedance matched to an air-filled waveguide interface. For the purposes of this description, the terms “air-filled,” “empty,” “vacuum-filled,” or “unloaded” may be used interchangeably to describe a waveguide structure. Conventional three-port waveguide switching circulators typically have one or more stages of quarter-wave dielectric transformer structures for purposes of impedance matching the ferrite element to the waveguide interface. The dielectric transformers are typically used to match the lower impedance of the ferrite element to the higher impedance of the air-filled waveguide so as to produce low loss. Thin adhesive bondlines are used to attach the transformers to the ferrite element and the waveguide structure, so they also provide a thermally conductive path from the ferrite element to the waveguide structure for transferring heat out of the ferrite element.
Previous patents have described approaches for achieving broad bandwidth through the addition of impedance matching elements. Broadband circulators have high isolation and return loss and low insertion loss over a wide frequency band, which is desirable so that the circulator is not the limiting component in the frequency bandwidth of a system. Broad bandwidth also allows a single design to be reused in different applications, thereby providing a cost savings. These previous approaches for achieving broad bandwidth generally involve the addition of quarter-wave dielectric transformers or steps in the height or width of the waveguide structure to thus achieve impedance matching the ferrite element to the waveguide port. For example, previous approaches have disclosed achieving impedance matching by providing a step or transition in the waveguide pathway. This technique eliminates the standard dielectric transformers, thereby eliminating a thermal path for conducting heat out of the ferrite element. This technique also relies on the presence of a significant gap or spacing between adjacent ferrite elements, increasing the size and weight of the structure. These methods all require impedance matching elements in addition to the ferrite element in order to achieve acceptable performance. Other approaches include changing the shape of the ferrite resonant structure to achieve broadband performance. However, these ferrite structures are restricted to fixed-bias applications with a single direction of circulation.
Referring now to FIG. 1, there is shown a top view of a conventional ferrite element. Although magnetizing windings are not shown, dashed lines 135 denote the apertures for the magnetizing windings. The apertures 135 for the magnetizing windings may be created by boring a hole through each leg of the ferrite element, for example. If a magnetizing winding is inserted through the apertures, then a magnetizing field may be established in the ferrite element, as would be evident to those possessing an ordinary skill in the pertinent arts. The polarity of this field may be switched, alternately, by the application of current on the magnetizing winding to thereby create the switchable circulator.
Resonant section 130 exists where the legs of device 101 converge inside the three apertures 135. As would be evident to those possessing an ordinary skill in the pertinent arts, the dimensions of resonant section 130 determine the operating frequency for circulation in accordance with conventional design and theory. The sections 140 of the ferrite element in the area outside of the magnetizing winding apertures 135 may act as return paths for the bias fields in the resonant section 130 and as impedance transformers out of the resonant section. Faces 150 of the ferrite element are located at the outer edges of the three legs.
Referring now to FIG. 2, there is shown a top view of a conventional single-junction waveguide circulator structure. FIG. 2 shows a ferrite element 101 with a quarter-wave dielectric transformer 103 attached to each leg. As shown in FIG. 2, the quarter-wave dielectric transformers 103 are generally much narrower than the ferrite element 101, which limits the ability of the quarter-wave dielectric transformers 103 in providing a thermally conductive path from the ferrite element 101 to the waveguide structure 100. A dielectric spacer 102 may be disposed on the top and bottom surfaces of ferrite element 101. Spacer 102 may be used to properly position the ferrite element in the housing and to provide a thermal path out of ferrite element 101 to the conductive (electrically and thermally) waveguide structure 100. Conventional circulators have minimized the diameter of this spacer for impedance matching purposes, and the diameter is generally smaller than the size of resonant section 130 discussed hereinabove. Generally, a smaller diameter spacer will provide more frequency bandwidth and a poorer thermal path. This opposing effect makes high power, broadband circulators difficult to achieve.
The conventional components described above may be disposed within the conductive waveguide structure 100, which is generally air-filled. For the purposes of this description, the terms “air-filled,” “empty,” “vacuum-filled,” or “unloaded” may be used interchangeably to describe a waveguide structure. Conductive waveguide structure 100 may include waveguide input/output ports 105. Ports 105 may provide interfaces, such as for signal input and output, for example. Empirical matching elements 104 may be disposed on the surface of conductive waveguide structure 100 to affect the performance. Matching elements 104 may be capacitive/inductive dielectric or metallic buttons that are used to empirically improve the impedance match over the desired operating frequency band.
Referring now to FIG. 3, there is shown a partial side view of a conventional single-junction waveguide circulator structure. As may be seen in FIG. 3, only one of the three legs of the ferrite element is shown. This view shows dielectric spacers 102 located between the walls of waveguide structure 100 and ferrite element 101. Adhesive materials are used to bond the dielectric spacers 102 to the waveguide structure 100 and to the ferrite element 101. As a result of the dielectric spacers 102 being much smaller in diameter than the legs of ferrite element 101, air gaps 110 exist above and below portions of the legs of the ferrite element. Air gaps 110 may be approximately one-third the height of the waveguide in the E-plane axis. Co-pending, commonly assigned patent application, U.S. non-provisional patent application Ser. No. 11/107,351 titled Latching Ferrite Waveguide Circulator Without E-Plane Air Gaps (the '351 application), incorporated herein by reference, describes implementations wherein the E-plane air gaps have been eliminated through the use of filler materials between the ferrite element 101 and the waveguide structure 100. The primary purpose of these filler materials is to suppress the high peak power breakdown effects such as arcing or multipactor. For broad bandwidth applications, these materials will generally have a low dielectric constant (less than 3), thereby preventing the use of the more thermally conductive dielectrics such as aluminum nitride, boron nitride, and beryllium oxide, which all have relative dielectric constants greater than 4.
The purpose of a ferrite circulator is to circulate RF power from one port to another while absorbing a minimal amount of the circulating power. All of the dielectric and ferrite materials in circulators absorb some power, but the majority of the power absorbed by a ferrite circulator is contained in the ferrite element due to the relatively high volume of the ferrite element 101 and the high electrical and magnetic loss tangents of the ferrite material. In conventional single-junction waveguide circulators, such as illustrated in FIG. 3, the ferrite temperature rise resulting from the power absorption is primarily dependent on the thermal resistance of the various paths from the ferrite element 101 to the thermally conductive waveguide structure 100. The waveguide structure 100 acts as a heat sink for the ferrite element 101, but the thermal paths between these two parts are limited in conventional circulators. These paths flow from the ferrite element 101 through adhesive bonds to either the dielectric spacers 102 or quarter-wave dielectric transformers 103 and on through adhesive bonds to the waveguide structure 100. The dimensions of the dielectric spacers 102 and quarter-wave dielectric transformers 103 are restricted by RF performance requirements rather than thermal requirements.
Accordingly, a need exits for a ferrite circulator that incorporates thermally conductive dielectric attachments in order to maximize the area of contact with the ferrite for improved heat transfer beyond the present art, thereby allowing ferrite circulators to operate at higher average microwave power levels.